The mechanism of cell nucleation and growth in foam injection molding with gas-counter pressure is investigated through molding experiments and numerical simulation. The molding data includes the visualization of cell nucleation and growth during foam injection molding as well as cavity pressure data. A numerical simulation is developed in this study to model the foam injection molding process with gas-counter pressure. The model is then validated using the experimental results. The effect of process conditions such as filling time, packing pressure and gas counter pressure are examined. The cell nucleation process is modeled using both the fitted nucleation model and the modified heterogeneous nucleation model and the effectiveness of each model is discussed. The importance of material properties such as surface tension, solubility and diffusivity on the bubble nucleation and growth models is also studied.